In the video compression field, the MPEG-1 standard has been used mainly for the video coding at rates around 1.5 Mbit/s, and is considered inappropriate for higher "broadcast quality" applications. To meet more demanding applications such as post-processing, and compression of sports and other challenging video sources, the draft MPEG-2 standard activity was initiated. The standard will provide for high quality video compression (typically at bit rates between 3-10 Mbit/s) and is expected to accommodate high definition television (HDTV) and be downward compatible with MPEG-1. While MPEG-1 assumes the source to be frame based, draft MPEG-2 attempts to take interlace effects into account and provide for a certain degree of adaptive frame/field processing to improve the compression.
The basic video unit in MPEG processing is a Group of Pictures (GOP) depicted in FIG. 1a. The GOP consists of:
(a) Intra (I) frames/fields which are coded purely spatially; PA1 (b) Predicted (P) frames/fields which use previously coded I (or P) frames/fields as predictors; and PA1 (c) Bi-directional (B) frames/fields which use the closest previously occurring I (or P) frames/fields and or the closest I (or P) frames/fields in the future as predictor.
In the draft MPEG-2 standard, there are basically six levels (from level 0 to level 5) of processing. The levels differ in the choice of picture structure (frame-based or field-based) and in the motion prediction mode, i.e., forward, backward or bi-directional prediction. Generally speaking, the low levels use field-structure processing and forward prediction (which requires less hardware complexity), while high level modes tends to use frame-structure processing and bi-directional prediction. The higher level modes can also accommodate some form of field-based motion estimation within a frame picture structure.
The draft MPEG-2 specification assumes implicitly that the processing level and the picture structure is fixed. Most proposed MPEG-2 motion estimation architectures operate in the frame-structure mode, obtaining frame motion vectors and in addition, separate motion vectors for the fields within the frame. However, the two fields within a frame cannot be used to predict each other. FIG. 1b clearly illustrates that every other field in a frame is separated in time by two field-periods from the closest field in the previous frame. In the case where fast motion is present in the video signal, the two-field interval can cause the displacement to exceed the capability of the motion estimator. It is therefore advantageous for each field to be predicted from the previously occurring field for such fast moving sources.
As expected, much effort has been expended in developing circuitry and methods for implementing MPEG-1 and its successor MPEG-2. U.S. Pat. No. 5,231,484, for example, describes a system for implementing an encoder suitable for MPEG-1 compression of video. Specifically, bit allocation for the pictures, adaptive DCT quantization, and pre-filtering strategies are covered by this patent.
U.S. Pat. No. 5,111,292 discloses an apparatus for separating compressed video data into a high priority stream and a low priority stream while U.S. Pat. No. 5,122,875 discloses both an apparatus for separating compressed video data into a high priority stream and a low priority stream and a method for transmitting, receiving and decoding these streams. U.S. Pat. No. 5,148,272, on the other hand, discloses circuitry for recombining, at the receiver, compressed video data which has been prioritized between high and low priorities.
U.S. Pat. No. 5,193,004 discloses a method for compressing even fields of interlaced video by creating a prediction for the even field using past and future odd fields, i.e., bi-directional field-based prediction. U.S. Pat. No. 5,212,742 discloses an architecture and a corresponding method for providing sufficient computing power for real-time decompression of moving video images. On the other hand, U.S. Pat. No. 5,212,549 discloses an apparatus as the receiver for detecting and concealing transmission errors in the compressed video data.
U.S. Pat. Nos. 5,185,819 and 5,146,325 disclose methods for reducing the timing latency with respect to image display on a receiver when the receiver is turned on or when the channel is changed. The '819 patent further discloses techniques for reducing the effect of errors, which is accomplished by compressing the odd fields of the video signal as one IBBP type sequence and the even fields of the video signal as a different IBBP sequence. The "I" fields of the even sequence do not follow the "I" fields of the odd sequence, instead these fields are offset by a distance equal to half the number of pictures in a single Group of Pictures.
U.S. Pat. No. 5,168,356 provides a method for segmenting encoded video data into transport blocks for signal transmission. U.S. Pat. No. 5,247,347 is related to a public switched telephone network configuration for video distribution in accordance with MPEG standards.
U.S. Pat. No. 5,227,878 discloses a method for compressing and decompressing interlaced video. Input video is analyzed and a decision is made to motion estimate and compensate a 16(H).times.16(V) block as an entity, or as two 16.times.8 frame blocks or two 16.times.8 field blocks. The coding mode yielded by the analyzer also affects the quantization table chosen for the DCT coefficients, the scanning order to the quantized DCT coefficients, and the choice of variable length code table which is used to further compress the scanned coefficients. All the stages of compression are made to be adaptive based on block type, e.g., intra/inter, frame/field, forward/backward predicted. The motion processing described in this patent is already covered by the draft MPEG-2 standard.
U.S. Pat. No. 5,241,383 discloses a method for bit allocation and quantizer adjustment to obtain good quality video at the specified bit rate.